Process and apparatus for treating biosolids from wastewater treatment

ABSTRACT

A process and apparatus for treating biosolids from biological wastewater treatment. The temperature of autothermal aerobic treatment of biosolids is controlled by sensing the oxidation/reduction potential of the solution and adjusting the amount of oxygen or biosolids supplied to the solution through a jet aerator ( 1 ) having a liquid header ( 3 ) for the recirculation of the solution and an air header ( 5 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/019,530, filed on Feb. 5, 1998, now U.S. Pat. No. 5,948,261 and acontinuation-in-part of application Ser. No. 09/233,532, filed on Jan.20, 1999, now U.S. Pat. No. 6,168,717.

FIELD OF THE INVENTION

The invention relates generally to a new process for the treatment ofbiosolids resulting from the treatment of biological wastewater streams.More particularly, the invention comprises an autothermal aerobicprocess for treating biosolids where the oxygen/reduction potential iscontrolled by adjusting the amount of shear generated through jetaeration devices and/or adjusting the amount of oxygen-contouring gasprovided to the process. The invention provides for a truly aerobicenvironment under which thermophilic microorganisms will thrive.

BACKGROUND OF THE INVENTION

Wastewater such as sewage streams generally have various naturallyoccurring and/or man-made contaminants, notably organic contaminants. Ina remarkable display of the versatility of nature, some naturallyoccurring microorganisms have the ability to consume these contaminantsfor their own life processes, thereby turning what is an undesirablepollutant into (for their purposes) a beneficial nutrient or foodsource. The wastewater treatment industry frequently capitalizes on theability of these microorganisms by using such microorganisms infacilities that treat wastewater streams to destroy the contaminants andbreak them down into basic compounds. Wastewater streams are fed intotanks or ponds that maintain conditions conducive to microorganismactivity. Typically, the microorganisms which consume the targetedcontaminants are mesophilic and thrive at temperatures in the range ofabout 25 to about 50 degrees Celsius.

The desired result of this type of wastewater treatment is thedestruction of organic contaminants, but a by-product of this type oftreatment is the production or increase of a biomass or biosolidscomprised of the microorganisms. The biosolids yield from waste watertreatment can range from about 0.1 pound of biosolids per pound ofbiological oxygen demand (BOD) removed to about 1 pound of bacteria perpound of BOD removed. A more typical range of biosolids yield is fromabout 0.3 pounds to about 0.6 pounds of bacteria per pound of BODremoved. Disposal of this biosolids may still be problematic, even aftermany contaminants have been consumed by microorganisms. One problemarises from the pathogenic nature of many microorganisms, such as theFecal Coliform group of organisms; although such microorganisms haveproven beneficial in consuming contaminants, they themselves may pose adanger to human health and are disease causing organisms. These includebut are not limited to certain bacteria, protozoa, viruses and viableheiminth ova. Regulations by states and/or the federal government imposerestrictions upon land disposal of materials containing pathogenicmicroorganisms. It is desirable to treat biosolids so that one caneasily and legally dispose of the biosolids on land or under ground.Suitably treated biosolids may even prove to have beneficial uses. Undercertain circumstances, it may be used as a soil conditioner orfertilizer.

Another problem with the biosolids may arise from the sheer volume ofbiomass generated. Costs associated with the production and disposal ofbiosolids include both capital costs and operating expenses, such asbiosolids disposal costs, trucking costs, material handling costs,management costs, and liability costs associated with disposal. Most ifnot all of these costs depend on the volume of biosolids at issue, and areduction in the amount of biosolids can make an economically unfeasibleoperation into a profitable one. Methods which will reduce the massand/or volume of biosolids to be disposed have significant commercialand environmental benefits.

Biosolids also contains other materials including microorganisms whichare not pathogenic in nature. Typically the biosolids includes a groupof microorganisms that thrive in what is generally referred to as thethermophilic temperature range. These thermophilic microorganisms arenormally not harmful to humans, and there are both aerobic and anaerobicbacteria that thrive within the thermophilic range. This invention isespecially interested in the aerobic microorganisms. Although thetemperature ranges for classification of bacteria varies somewhatdepending upon who is describing the range, thermophilic activityusually takes place within the range of from about 45° C. to about 70°C. In contrast, pathogenic bacteria usually thrive within what isreferred to as a mesophilic range which is from about 25° C. to about37° C. or the normal body temperature of humans, and may begin to die atabout 38° C.

Therefore, various methods have been proposed and practiced for treatingthe biosolids that results from treatment of wastewaters. Biosolids maybe treated aerobically or anaerobically, with different microorganisms,conditions and results. Among the methods of biosolids treatment isautothermal thermophilic aerobic digestion (“ATAD”). ATAD capitalizes onthe presence of materials in the biosolids such as naturally occurringmicroorganisms which are not pathogenic or harmful to humans but whichwill kill pathogenic microorganisms. Typically, these are thermophilicmicroorganisms which thrive at temperatures of from about 45° C. toabout 70° C.

A preferred temperature for thermophilic microorganisms is approximately65° C. When this preferred temperature is maintained during thetreatment of a wastewater biosolids, the reaction time for destructionof mesophilic microorganisms at 65° centigrade for purposes of meetinggovernmental regulations is approximately three quarters of an hour, asestablished by the Environmental Protection Agency's Standards for Useand Disposal of Sewage Biosolids, 40 CFR, Part 503. Three hours is aneasily obtained processing time for most biosolids treatment facilities,since biosolids, is often pumped once every twenty four hours from thewaste water treatment plant.

In a typical ATAD process, biosolids resulting from wastewater treatmentis treated in a reactor, which operates at a temperature in thethermophilic range, i.e., from about 45° C. to about 70° C. Temperaturesabove the above this range do not allow the thermophilic microorganismsto thrive and may even result in their destruction. Within thistemperature range, thermophilic microorganisms are active in an aerobicprocess where they consume oxygen, which must be provided in thesolution.

An advantage of an aerobic process using thermophilic microorganisms isthat their use of oxygen is an exothermic reaction. The heat released asa result of this reaction raises the temperature of the biosolidssolution. As the temperature rises above the mesophilic range,mesophilic microorganisms are killed and consumed by thermophilicmicroorganisms. It has been estimated by others that 9000 BTUs may bereleased for every pound of volatile suspended solids destroyed. Theinterrelated cycle processes in which exothermic reactions triggeradditional exothermic activity by thermophilic microorganisms results inan autothermal process and thereby creates an autothermal environment byvirtue of the maintenance of relatively high temperatures.

Pathogens could also be destroyed through the direct application of heatfrom an outside heat source to the biosolids solution. By directlyheating the biosolids to temperatures that are inhospitable formesophilic microorganisms, these pathogens may be killed. However, thistype of treatment (without the action of thermophilic microorganisms) iscostly and wastes energy, since the amount of heat that must be directlyapplied to raise the temperature of the biosolids mass is substantial.

A major challenge in operating an aerobic biosolids treatment process isto keep the process sufficiently aerobic by meeting or exceeding theoxygen demand while operating at the elevated temperatures in whichthermophilic bacteria thrive. One reason why this is difficult is thatas the process temperature increases, the saturation value of theresidual dissolved oxygen decreases. That is, a higher temperatureresults in less oxygen remaining in the biosolids solution. Anotherreason is that the activity of thermophilic microorganisms increaseswith higher temperature. This higher activity results in increasedoxygen consumption by the microorganisms. Hence, greater amounts ofoxygen must be imparted to the biosolids solution.

Another major challenge is to operate the process in an autothermalcondition while still maintaining some control over the operatingtemperature. In an autothermal process, the process operates at atemperature higher than ambient without adding heat or by adding lessheat than would ordinarily be needed to maintain the process at thattemperature. In the biosolids treatment industry, autothermal processescapitalize on the exothermic nature of the action of the thermophilicbacteria in breaking down and consuming mesophilic bacteria or otherorganic compounds. The use of autothermal processes can obviate the needfor external heat supply. However, it is still desirable or necessary tohave some means of controlling the temperature of the process.

The need to control temperature has been previously identified anddiscussed in U.S. Pat. No. 5,587,081, which discloses a method ofcontrolling temperature by varying the proportion of fresh air versusrecycled air injected into the biosolids. By increasing the amount offresh cool air introduced, the reactor is cooled. However the inventorbelieves it is important to use fresh air in the injection processbecause recycled air is not as effective in providing oxygen forthermophilic bacteria to thrive. The process described in U.S. Pat. No.5,587,081 does not appear to take into account the fact that recycledair, although warmer than fresh air, has less oxygen and will generateless exothermic reaction and heat from the thermophilic microorganisms.The recycled air has a lower content of oxygen than is found in ambientair This results in less oxygen being imparted to the biosolids solutionby the recycled air. Although at first glance, it may appear that theeffect of the reduced oxygen content is minimal because the reduction inoxygen may be only a few percent, in practice the reduced oxygen contentresults in insufficient oxygen being imparted to the solution to createa truly aerobic environment for the aerobic microorganisms to thrive.

Various apparatus and methods have been used to inject an oxygencontaining gas stream into a biosolids solution. For example, spargers,diffusers and aerators of various designs and configurations have beenused. It is the inventor's opinion that the best apparatus to deliverthe necessary oxygen is the aeration jet. One such aeration jet has beendeveloped by Mass Transfer Systems, Inc., (“MTS”) 100 Waldron Road, FallRiver, Mass. MTS has been purchased by Waterlink and have been put underits biological wastewater systems division, which lists its address as630 Currant Road, Fall River, Mass., USA 02720. A product brochure byMTS is enclosed herein and incorporated by reference. By using theaeration jet, it is possible to create finer air bubbles along withhigher shear which results in greater introduction of oxygen into thebiosolids solution. There are many other advantages associated with theaeration jet, including better mixing. As the biosolids treatment occursand mesophilic bacteria are broken down, carbon dioxide, water andammonia (as well as other organic compounds) are produced when theprotoplasm within the cell is released into the biosolids solution. Theammonia raises the pH of the solution and causes a noxious odor.Additionally, cell breakdown results in foam. It is desirable to havesome means to treat odor and foam.

A typical method of controlling foam has comprised breaking the walls ofthe foam bubbles by manual or a physical means. For example, somereactors have employed one or more cutting blades rotated by a motor.The blades spin through the foam layer, thereby rupturing foam bubbles,converting the foam back into a liquid. There are disadvantages to thisapproach for controlling foam, including maintenance and energy costsand efforts, particularly for a high rpm motor. Furthermore, the cuttingblades may erode over time and require periodic replacement. Anotherdisadvantage is that the motor that rotates the cutting blades istypically placed at the top of the reactor (outside the biosolidssolution and the foam). However, the heat that can build up at the topof the reactor may shorten the life expectancy of the motor.

SUMMARY OF THE INVENTION

The inventive process has been referred to by its inventor as theTHERMAERθ Process. The invention provides a method for controlling thetemperature of an autothermal process by adjusting the flow rate(s)through a jet aeration nozzle of circulated biosolids solution and/oroxygen-containing gas, thereby adjusting the rate of exothermic reactionfrom the interaction of oxygen with aerobic thermophilic microorganisms.The mechanism by which the biosolids flow rate and/or gas flow rateaffects the reaction rate is through the amount of shear produced as thebiosolids solution mixes with the oxygen-containing gas stream in thejet aeration nozzle. A higher amount of shear induces more reactions bythe thermophilic organisms, thereby producing more heat. Lowering thebiosolids flow rate and/or the gas flow rate results in less shear,which in turn induces less exothermic reaction by the microorganisms.

By maintaining an autothermal, truly aerobic treatment environment,numerous process advantages ensue as well as a better digested biosolidsproduct. Objects of the present invention include significantly reducingthe volatile solids in the biomass, reducing the total mass of biosolidsand producing a stabilized material suitable for land disposal. Anotherobjection of the present invention is to create and maintain a trulyaerobic environment for the treatment of waste water biosolids. A trulyaerobic biological process has sufficient oxygen present to support theliving organisms' respiration rates and does not allow an anoxiccondition to occur.

The THERMAERθ Process which incorporates the present invention involvesthe surprisingly effective use of lower air flows and higher liquidflows. Counterintuitively, the use of a lower airflow can actuallyincrease, the amount of oxygen imparted into solution. It is believedthat using a lower air flow process results in the injection ofextremely fine bubbles into the treatment solution and higher surfacerenewal of the solution.

The present invention facilitates the treatment of biosolids in anautothermal process by removing a high percentage of water andincreasing the organic concentration in a biosolids thickening processthat precedes introduction of the biosolids into the treatment reactor.By thickening the biosolids, the volume of the biosolids solution may besignificantly reduced, thereby enabling greater temperature controlthrough the use of liquid flow rate.

The inventive process may be tailored to virtually any individualapplication. Different industrial plants have different product mixeswith different sets of constituents. The complexity of the organicchemistry can vary from short chain molecules that are readily brokendown to long chain molecules that are difficult to break down. TheTHERMAERθ Process has the flexibility to deal with varying plantconditions and can operate at varying liquid depths, at varyinghydraulic and solids retention times and operate as a single tankreactor or multiple tank reactors.

In the preferred embodiments of the present invention, the temperatureof a truly autothermal aerobic process is controlled through a variablefrequency drive on a jet motive pump which circulates biosolids throughthe jet aeration device into the reactor. Reactor temperature iscontrolled by varying the force in which the biosolids solution iscirculated or re-circulated into the reactor through an aeration jet orother suitable means. In other embodiments, reactor temperature iscontrolled through the air pump used to control the flow rate ofoxygen-containing gas through the jet aeration device.

In the present invention, the perceived problem of foaming caused by thetreatment process is turned into an advantage. The inventor has notedthat foam can act as an insulator between the biosolids solution and theair in the top of the reactor. In a typical reactor, the reactor isvented to the atmosphere so that it is not under pressure. As a result,the temperature of the air in the reactor is affected by the temperatureof outside the reactor; in some cases, the temperature of the air in thereactor may be the same as the ambient temperature outside. Byrefraining from destroying all the foam bubbles, it is possible to usethe foam as an insulator between the biosolids solution and the air inthe reactor. Preferably, a foam control system is operated to maintain alayer of foam having a depth of from about four to about eight feet,preferably about six feet.

The inventive process may be used to treat a biosolids solutioncomprised of the products of waste water treatment and thermophilicbacteria capable of digesting mesophilic bacteria. The process comprisesthe steps of (a) thickening biosolids solution before it first enters abiosolids treatment reactor to a concentration of from about 3% to about6% solids; (b) mixing a portion of biosolids solution with anoxygen-containing gas stream using a jet aeration device; (c) injectinga mixture of the oxygen-containing gas and biosolids solution into areactor at, a flow rate which introduces sufficient oxygen into thestudy solution so that the treatment environment is substantiallyconstantly aerobic; and (d) controlling the temperature of the biosolidssolution by adjusting an amount of shear generated through the jetaeration device. In some embodiments, the amount of shear (and thetemperature of the biosolids solution) is controlled by adjusting theliquid flow rate of biosolids through the jet aeration device whilekeeping the flow rate of oxygen-containing gas constant. In mostembodiments the portion of biosolids solution mixed with,oxygen-containing gas in the jet aeration device will be recirculatedbiosolids that has been removed from the general biosolids solution inthe reactor and pumped through the jet aeration device.

The inventive process may also include the step of wasting a portion oftreated biosolids wherein the wasting step is performed in the sameapparatus in which the thickening step is performed. “Wasting” is a termused in the industry to mean dewatering biosolids prior to its disposal.

Alternate embodiments of the present invention comprise an apparatus forautothermal aerobic treatment of wastewater treatment biosolids. Thatapparatus comprises a means for concentrating a wastewater treatmentbiosolids to a concentration of at least about 3 percent solids. Amongthe suitable means, for concentrating the biosolids solution are ahorizontal solid bowl-decanting centrifuge, a gravity belt, a rotarydrum thickener, dissolved air flotation, gravity settling, or theapplication of evaporative heat. The apparatus also comprises a reactorhaving an inlet from said concentrating means for the introduction of atleast one biosolids and a jet aeration device affixed to the bottom ofthe reactor.

The jet aeration device comprises an air header having one or moreopenings through which a gas transported through the air header may exitthe air header; a liquid header running parallel to and/or concentricwith the air header and having one or more openings through which abiosolids solution transported through the liquid header may exit theliquid header; an outer nozzle extending from the liquid header andhaving an opening on its side; an inner nozzle from the liquid headerand contained within the outer nozzle; one or more air passageconnections from the air header to the outer nozzle which connects theair header to the liquid header and provides a closed path for air fromthe air header to travel to the outer nozzle and enter the outer nozzlethrough its side opening; and liquid from the liquid header are mixed inthe outer nozzle. The apparatus comprises an air distribution pipeconnected to the air header, which provides an oxygen-containing gasfrom outside the reactor; and a liquid outlet located at or near thebottom of the reactor, which allows biosolids solution to exit thereactor. The apparatus may optionally include a motive pump connected tothe liquid outlet such that biosolids solution is withdrawn from thereactor by the motive pump. Attached to the motive pump is a motive pumpconduit that leads from the motive pump to the liquid header such thatbiosolids solution is pumped through the conduit into the liquid headerand forced through the inner nozzle by force of the motive pump.

The present invention may also include apparatus for automaticallysensing and controlling the temperature in the reactor by adjusting therate at which liquid is circulated into the reactor through the jetaeration device. Alternatively or additionally, the present inventionmay include apparatus for automatically sensing and controlling theoxidation/reduction potential (“ORP”) of the solution in the reactor, byadjusting the rate at which biosolids, liquid or gas is circulated intothe reactor. This apparatus will typically include a temperature sensorand/or an ORP sensor within the reactor and means for automaticallycontrolling the motive pump and/or air blower. In other embodiments, aphysical property other than temperature or ORP may be sensed ormonitored. Suitable means for automatically controlling include aprogrammable logic controller (“PLC”), a computer, analog signal or amicroprocessor. This automatic control means is operatively attached tothe temperature sensor and/or ORP sensor and the motive pump and/or airblower such that based on the temperature of the biosolids solution inthe reactor as measured by the temperature sensor, and/or the ORP of thesolution as measured by the ORP sensor, the automatic control means willinstruct the motive pump and/or the air blower to adjust the flow ofbiosolids solution and/or the air flow in order to adjust the amount ofsolution and/or oxygen-containing gas into the reactor. This can effectthe temperature and/or ORP of the biosolids solution in the reactor.

Apparatus embodying the present invention may also comprise a secondarycooling system, which comprises a cooling jet nozzle located in thereactor above the level of the jet aeration device; and a coolingconduit extending from the motive pump conduit to the cooling jet nozzlesuch that biosolids solution traveling through the cooling conduit losesheat to the surrounding environment.

In one embodiment of the present invention, the reactor holds abiosolids solution having a depth of at least about 24 feet. Anotherbenefit of the present invention is it can be used in larger reactors.Because the invention can be used in larger reactors, the residence timeof biosolids in a reactor can be increased so that biosolids may remainin a single reactor throughout the entire treatment period

As discussed above, the foam created during the treatment process can beused to advantage, as an insulator between the biosolids solution andthe air in the reactor. Nonetheless, a reliable foam control system isnecessary to maintain a layer of foam at a desirable depth and preventan excess of foam from escaping from the reactor.

In a further refinement of the THERMAERθ, Process, an inventive methodand apparatus for foam control system has been developed. This methodand apparatus may be used in conjunction with or separately from theother steps and apparatus of the THERMAERθ, Process described herein.

In one embodiment, the method comprises the additional or separate stepsof generating a layer of foam on top of the biosolids solution,transferring a portion of the layer of foam from on top of the biosolidssolution into the biosolids solution through a foam transfer pipe, andconverting at least some of the portion of the layer of foam into liquidduring transfer through the foam transfer pipe. The foam transfer pipepreferably includes a static mixer or other means that impart a dynamicmixing action to the foam, thereby rupturing or collapsing foam bubbles.Dynamic mixing action is action that imparts turbulence or energy thatcauses foam bubbles to collapse or rupture. One way to impart dynamicmixing action is to cause the fluid to have turbulent flow; another wayis to mix the fluid or cause the fluid to move in a swirling motion.Alternately, the method may comprise the steps of transferring a portionof the foam from on top of the solution into the solution through thefoam transfer pipe; mixing the foam in the foam transfer pipe so that atleast some of the portion of foam is converted to liquid while passingthrough said foam transfer pipe; and drawing at least a portion of foam(which may be converted to liquid) by suction through at least a portionof the foam transfer pipe. The source of the suction may be an outernozzle of a jet aeration system similar to those described herein,except that one outer nozzle is not connected to an air header; instead,it is dedicated to the foam transfer pipe. As fluid flows through theinner nozzle, it generates a vacuum or draw in the outer nozzle thatpulls or sucks liquified foam from a foam transfer pipe that is fluidlyconnected to the side of the outer nozzle.

The foam control apparatus is preferably used in connection with theATAD treatment reactor comprising a jet aeration system as describedabove. The foam control apparatus comprises a foam transfer pipe havinga top opening, a bottom opening and an internal surface, wherein saidtop opening is at least above an anticipated level of a solution (forexample, a biosolids solution), the bottom opening is at least below theanticipated level of the solution and is fluidly connected to a suctionsource. The suction source is preferably an outer nozzle of a jetaeration device that is dedicated to the foam transfer pipe or anotherventuri device. The foam transfer pipe preferably has a static mixerdisposed therein. The static mixer may be affixed to the internalsurface of the foam transfer pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of a jet aeration nozzle used in theinvention.

FIG. 2 shows a biosolids treatment system as a integrated part of awaste water and biosolids treatment facility.

FIG. 3 shows a biosolids treatment reactor and associated processequipment for the biosolids treatment process.

FIG. 4 shows a temperature correlation chart for a hypotheticalinstallation of the invention, wherein the appropriate pump speed is setfor a given temperature of the biosolids solution.

FIG. 5 shows foam control equipment for a treatment reactor, including ajet aeration system and a foam transfer pipe.

FIG. 6 shows an approximation of the estimated oxygen demand in anaerobic treatment of biosolids.

DETAILED DESCRIPTION OF DRAWINGS AND PREFERRED EMBODIMENT

The inventor contemplates that preferred embodiments of the inventionwill involve the use of a jet aeration device. FIG. 1 shows across-section of a jet aeration device 1. The device includes a liquidheader 3 and an air header 5. The liquid header and the air header maybe longitudinal, circular or radial in shape. The liquid header 3transports a liquid such as an untreated or recycled biosolids stream.The liquid header 3 has a relatively small opening or inner nozzle 7which allows the liquid to exit from the liquid header into an outernozzle 9 or other conically shaped structure. The air header 5 also hasan air header opening 11 which allows the gas to exit through an airpassage way 13 into the same outer nozzle 9. In the outer nozzle 9, thegas and the liquid mix so as to create a shear that induces theexothermic action of the thermophilic microorganisms. The air header 5carries a gas such as an oxygen-containing gas useful for an aerobicprocess. Typically the source for the oxygen-containing gas is ambientair outside the reactor; however, the gas may be obtained from anysource provided that it contains sufficient oxygen for a truly aerobicprocess. The inventor believes that such a gas should have at least theconcentration of oxygen found in ambient air (approximately 21.9 percentby volume).

The jet aeration device 1 must have the ability to transfer a highamount of oxygen-containing gas into a high suspended solidconcentration while completely mixing the reactor Contents. When theliquid is mixing with the air, a shear is produced. The inventorcontemplates that the flow rate of biosolids solution through the innernozzle of a jet aeration device would be in the range of from about 30feet/second to about 50 feet/second.

FIG. 2 is a schematic of an integrated treatment system for the initialtreatment of wastewater and the secondary treatment of the biosolidsresulting from that initial treatment. Wastewater is treated in anaeration basin 17, resulting in the production of an effluent comprisingbiosolids and water. The biosolids typically has various mesophilic andthermophilic microorganisms The effluent is transported to a clarifier19 which separates a portion of biosolids solution to be disposed of.The clarifier 19 can accept biosolids from a number of sources and ofseveral different make-ups. After the biosolids solution leaves theclarifier 19, a portion of the biosolids may be returned to the aerationbasin 17 to insure that it has sufficient amounts of activemicroorganisms to devour the waste water contaminants. Another portionof the biosolids will be sent for treatment according to the presentinvention prior to ultimate disposal. To the biosolids that is to betreated, one may choose to add one or more charge neutralizing polymersfrom polymer containers 25 to allow for greater flocculation of thebiosolids. Alternately, a thickening polymer may be added to thebiosolids solution as it is being concentrated.

The portion of biosolids to be treated and disposed is transferred to ameans for concentrating the biosolids to a higher solids concentration.Any suitable means may be used for concentrating the biosolids solution.One preferred means is a horizontal solid bowl-decanting centrifuge 21.Other means include a gravity belt, a rotary drum thickener, a DAP,gravity settling, or the application of evaporative heat. The centrifugemay be controlled by a separate PLC that can be programmed to operate onmore than one process curve. A PLC can monitor the torque that isproduced on the biosolids cake and adjust the scroll speed accordinglyto achieve consistent results. Typically, the solids content of abiosolids prior to treatment is from about 0.5% to about 1.5% with avolatile solids content of from about 65% to about 90%, with 80%preferred. The inventor has found it desirable to concentrate thebiosolids solution to a solids content of from about 3% to about 6%,with 5% to 5.5% being preferred, prior to treating the biosolidssolution. The inventor has found it necessary to thicken the biosolidsto a solids content of from about 3% to about 6% in order to run anoptimal autothermal treatment process.

From the centrifuge, the biosolids solution is transferred to atreatment reactor 23 via a feed line 24. After treatment, digestedbiosolids may be removed from the reactor 23 via a removal pipe 27 whichtransfers the biosolids to the same or a different concentrating means.Preferably the same concentrating means 21 is now used to reduce thewater content of the treated. A coagulant tank 29 for a coagulant suchas ferric chloride may also be provided and operatively connected to theremoval pipe 27 so that coagulant may be introduced into digestedbiosolids. From the concentrating means, the biosolids is removed by aconveyor system and sent for disposal. Biosolids will generally beremoved from the reactor on a batch per day basis. The volume of biomassremoved from the reactor will typically be about the same as the volumeof biosolids to be introduced into the reactor for treatment that day.Using the same concentrating means to remove water before and aftertreatment in the reactor may achieve substantial savings on the cost ofcapital equipment.

FIG. 3 shows a treatment reactor 33 in greater detail and associatedprocess equipment for the biosolids treatment process. The reactor 33contains a biosolids solution that is treated according to the inventiveprocess. The reactor contains an arrangement or unitary sequence of jetaeration devices 35 (as described above and shown in cross-section inFIG. 1) affixed to the bottom floor of the reactor. Preferably, theouter nozzles of the jet aeration device 1 point around the reactor.

A motive pump 37 may be employed to circulate the biosolids solutionthrough the liquid header 35 a (shown in cross-section in FIG. 1 asliquid header 3). The motive pump 37 pumps the biosolids solutionthrough the liquid header 35 a of the jet aeration device 35. It ispreferred that the motive pump 37 have a variable frequency drive 39 orother means of varying the liquid flow, which may vary the forcegenerated by the motive pump 37, thereby varying the flow rate andpressure of the biosolids solution through the liquid header 35 a andthrough the outer nozzle 9 (shown in FIG. 1). A varying flow rate isdesirable because it is desirable to have the ability to control theliquid flow rate through the liquid header 35 a in order to control theamount of energy generated by shear.

Air or another oxygen-containing gas is introduced from outside thereactor through an air distribution pipe 41 whose upstream end isconnected to one or more air blowers 43 which blow air or anothersuitable oxygen-containing gas through the air distribution pipe 41. Theair distribution pipe 41 transports air or gas to the air header 35 b(shown in FIG. 1 as air header 5). The air header 35 b may be detachedfrom or affixed to the liquid header 35 a.

Different amounts of energy are needed at different phases of thetreatment operation. During the start-up phase, when a batch ofuntreated biosolids solution is first introduced into the reactor 17, alarge amount of shear is required to begin the exothermic reaction tothe extent necessary to bring the reactor contents up to the operatingtemperature, which is from about. 55 degrees Celsius to about 65 degreesCelsius, alternately about 63 degrees Celsius. The motive pump 37 isoperated at the speed necessary to obtain a desired shear due to a highliquid flow rate until the viscosity of the biosolids solution hasreached a normal operating level. At that time, the motive pump 37 isslowed so that the liquid flow rate is sufficient to sufficiently mixthe contents of the reactor so that exothermic reactions continue and toinject oxygen-containing gas into the biosolids solution flowing fromthe liquid header. After start-up, the biosolids solution in the reactor33 should operate at a self-regulating autothermal temperature at whichthe heat provided by exothermic reactions is in equilibrium with theheat lost to the outside environment.

A temperature sensor may be provided inside the reactor to measure thetemperature of the biosolids solution and send a signal to a suitablyprogrammed PLC connected to the motive pump. An indirect method ofsensing the temperature of the biosolids solution is to measure theoxygen reduction potential (“ORP”) of the biosolids solution. ORP isdirectly related to temperature. A high negative value for ORP indicatesthat the biosolids solution has a high oxygen up-take requirement andthat the speed of the motive pump should increase to provide more oxygenand raise the temperature. Using an ORP sensor instead of a directtemperature sensor would require an additional set of data pointsconnecting ORP values to temperatures for a given system.

A PLC may be programmed so that it will speed the motive pump and/or theair blower if the temperature of the biosolids solution drops below theminimum desirable temperature. Alternatively, a PLC may be programmed sothat it will speed the motive pump and/or the air blower if theoxygen/reduction potential drops below the minimum desirable potential.Alternatively, a PLC may be programmed so that it will speed the motivepump and/or the air blower if some monitored physical property reaches apredetermined value. The effect of speeding the motive pump and/or theair blower will be to increase the liquid flow rate and/or the airflow-rate, respectively, in the jet aeration device and to increase theshear, thereby increasing the exothermic reaction by the thermophilicorganisms. This increased reaction will provide additional heat, raisingthe temperature.

Conversely, if the temperature of the biosolids solution rises above themaximum desirable temperature, the PLC will send a signal to the motivepump, which slows the motive pump. This will reduce the liquid flow rateand shear, thereby reducing the level of exothermic reaction and heatproduced thereby. In this way, the inventive system automaticallymaintains the temperature of the biosolids solution within apredetermined range through the liquid flow rate and/or the airflow-rate. The motive pump and the air blower is operated within apredetermined range of speeds that is set by the flow and pressure curvefor each individual application.

Parameters for a PLC or other means of automatically controlling themotive pump must be uniquely generated for each particular installationbecause each installation has unique reactor and conduit configurations,biosolids concentrations, and pump characteristics. Typically, oneidentifies the corresponding temperature and pump parameters by plottingempirically determined temperature data points on the performance curvefor the motive pump supplied by the seller of the pump. The highestdesired temperature is plotted at the lowest desired RPM andcorresponding flow rate for a given resisting pressure, or head. Forexample, for a model 3180/3185 pump, a minimum desired temperature ofabout 135 degrees F might be plotted at about 900 RPM, and the maximumdesired temperature of about 155 degrees F might be plotted at about 500RPM.

After the reactor contents have reached a desirable temperature, anappropriately programmed PLC device will cause the variable frequencydrive to maintain the motive pump at a constant rate. During certainoperating conditions, the exothermic reactions may create too much heat.If the temperature of the biosolids solution is too high, thethermophilic microorganisms may not thrive or may be killed. Otherpossible adverse effects of having too high a temperature are excessivefoam or odor. To account for the possibility that the reactor's normaloperating temperature is normal that the desired operating temperature,the present invention provides a secondary cooling system which iscomprised of a fluid by-pass which allows re-circulated biosolidssolution to surrender heat.

An ORP sensor may be provided inside the reactor to measure theoxygen/reduction potential of the biosolids solution in millivolts orother units of electric potential difference. In effect, the ORP sensormeasures the oxygen level in the reactor. Oxygen level is an importantparameter in an aerobic system, and it may be effected by differentoxygen demands at different times within the solution. This isparticularly true during or shortly after the feed cycle is begun, whenthe oxygen demand in the reactor increases.

FIG. 6 is a graph showing an approximation of the estimated oxygendemand in an aerobic treatment of biosolids. Thus, a system thatincludes an ORP sensor is especially able to sense those times whenoxygen demand is higher than normal and adjust the oxygen supply so thatthe solution will remain truly aerobic. In at least some current system,the oxygen supply remains consistent through the treatment. One objectof the present invention is to provide an oxygen delivery curve that isclose or identical to the oxygen demand curve, so that the amount ofoxygen supplied is the same as or close to the amount of oxygen demandedin the treatment.

In a system that includes an ORP sensor, when oxygen demand increases,the millivolt reading on the ORP sensor decreases and may go intonegative numbers. The ORP sensor periodically sends a signal to a PLC,which continually monitors the input signal. In one embodiment, the PLCwill be programmed to determine an average trend line of the millivoltreading over a period of time, perhaps over the period of five minutes.The average trend line will be compared to a preset value thatcorresponds to a default setting of the motive pump(s) and/or theaeration blower(s). The present value can be determined and set on thePLC based on the pump curve of the jet motive pump(s) and the aerationblower(s).

Periodically the PLC will adjust the variable frequency drive for themotive pump(s) and/or the variable frequency drive for the airblower(s). A normal operating mode or minimum frequency for the variablefrequency drive will be preset into the PLC. The normal operating modecan correspond to any reading on the ORP sensor, but will typicallycorrespond to a value between 0 and 20 millivolts. The PLC will respondto the ORP sensor in an inverse fashion, that is, as the average trendline millivolt reading decreases, the frequency of the variablefrequency drive will increase. The adjustment can occur independentlywith the pump(s) and blower(s) or simultaneously. This procedure willcontinue until the process stabilizes or the frequency is at 100% of thepreset maximum pump and blower setting. The system will continue tooperate at 100% capacity until the average trend line millivolt readingof the ORP sensor goes above the preset point typically between 0 and 20millivolts. As the reading reaches or exceeds the preset point, thecontroller will decrease the frequency on the variable frequency driveof the blower. As stated earlier, the adjustment can occur independentlywith the pump(s) and blower(s) or simultaneously. This procedure willcontinue until both the pump, and blower frequency setting are at thenormal operating set points or minimum set points. The controller willcontinue to monitor the average trend line millivolt reading for theremainder of the daily feed cycle and make the appropriate adjustmentswhile waiting for the next feed cycle to begin.

A major benefit of a control scheme comprising an ORP sensor inventivecontrol scheme is energy efficiency. The control scheme supplies therequired horsepower to increase oxygen supply when necessary andconserves energy when the demand is lower. Another extremely importantadvantage of this control scheme is that it may prevent sulfur compoundsfrom reducing and producing unwanted and very foul odors in addition tokeeping the production of unwanted volatile fatty acids to a minimum.

The secondary cooling system is generally a liquid by-pass thattransfers the reactor contents above the foam layer and through ajet-cooling nozzle. This action exposes the reactor contents to theatmosphere above the foam layer, thus causing the reactor contents torelease heat. The secondary cooling system may be include a conduit 45located outside the reactor which routes biosolids above the foam layer.This conduit 45 may attached to the pipe leading from the motive pumpback into the reactor. An actuated valve 46 may be placed so that thebiosolids solution's access to the conduit 45 is controlled, perhapsthrough the PLC based on the reading of the temperature sensor. Thesecondary cooling system may also comprise a cooling jet nozzle 47 atthe downstream end of the cooling conduit. The cooling jet nozzle 47injects the biosolids solution back into the reactor 33 at apredetermined location or height.

In one embodiment of the present invention, the air blowers 43 willtypically operate at a constant volume. It is believed that greaterprocess control is achieved by maintaining the air flow rate constantwhile varying the liquid flow rate to control temperature. In otherembodiments, the air flow may be varied as a method of controllingtemperature.

The present invention uses a lower air rate and a w higher liquid flowrate to create an extremely fine bubble and a high shear factor. Forexample, whereas a typical air flow may be as high as 80 scfm, thepresent invention employs an air flow rate of approximately 10 to 25scfm, alternately 16 to 20 scfm. Whereas a typical liquid velocity-maybe about 30 to 33 ft/s, the present invention generally employs a liquidvelocity of about 40 to 55 ft/s. The result of the lower air rate andthe higher liquid rate is an extremely fine bubble and a high shearfactor.

Volatile suspended solids are live cells. The dead cells cause aputrescible odor. The present invention may be used with a reactor ofany height or with a biosolids solution of any depth. However, it ispreferred that the biosolids solution be at a depth of at least about 10feet, alternatively at least about 24 feet, alternatively less thanabout 30 feet. It is believed that depths of about 24 feet are optimalbecause oxygen transfer increases with increased depth, as theoxygen-containing gas is released into the biosolids solution atincreased pressures. The maximum liquid depth of the biosolids solution(shown for general illustrative purposes in FIG. 3 as a line 53) forexisting apparatus is set by the pressure limitation of the air blowersand the mixing limitation of the tank geometry. The minimum liquid depth(shown for general illustrative purposes in FIG. 3 as a line 55) isgenerally no lower than the center line of the positive. pressure liquidpipe 51 leading from the motive pump 37 to the reactor 33.

In some preferred embodiments of the invention, foam is controlled bymeans other than physically contacting the foam to rupture its bubbles.Instead, foam is controlled in ways that do not physically contact thefoam yet still rupture an appropriate amount of foam bubbles. One suchway is the use of a sonic horn 49 to rupture foam bubbles through sonicwaves of an appropriate frequency. The horn may be sounded atappropriate intervals so that sufficient foam remains to cover thebiosolids solution but the danger of foam spilling over the top of thereactor is minimized. The sonic horn may be activated and controlled bya timer and a solenoid valve. In one embodiment of the inventiveprocess, the sonic horn is activated for approximately five minutes withoff-intervals ranging from between about ten minutes to about 55minutes, preferably about 25 minutes.

The reactor 33 typically also has additional features which do assist inoperation. The reactor 33 may have an overflow outlet 57 through whichexcess foam may escape and be diverted to a foam containment area orcontainer. The reactor 33 typically has vent 59 which allows the reactorvolume to remain at atmospheric pressure and an off-gas outlet 61 at ornear the top of the reactor 33 which is attached to a source of partialvacuum, thereby pulling off-gas out of the reactor 33. The off-gas isremoved at a rate that exceeds the incoming volume of air andcompensates for the air expansion that occurs from heating. The off-gas,which may have a foul odor, can be routed to the aeration basin 17(shown in FIG. 2) where the contaminants which cause the foul odor caneither be solubilized through pH and temperature reduction or adsorbedby the biosolids microorganisms and utilized for food. The reactor mayalso have a foam level detector 63 that will shut down the aerationblower 43 if the foam reaches too high a level.

After treatment in the reactor, the biosolids solution or a portionthereof is removed and transferred to the concentrating means so that itmay be dewatered. After this concentrating step, the biosolids may bedisposed.

FIG. 4 is a chart which correlates the temperature of the biosolidssolution to the speed of the motive pump, the velocity of biosolidssolution through the inner nozzle of the jet aeration system and thetotal dynamic head, which is a measure of pressure against which the jetaeration device injects biosolids solution into the reactor. The chartin FIG. 4 could be provided to a programmer to program a PLC to run themotive pump at the speed (in RPM) specified for each of the giventemperatures. Based on this chart, the PLC would make the motive pumprun at a speed of 770 RPM if the temperature sensor measured a reactortemperature of 135 degrees F.

Although FIG. 4 is useful to show the relation between the plottedparameters, it may or may not be appropriate for a given installationdue to the unique reactor characteristics and configurations andbiosolids concentration associated with each installation. FIG. 4 is ahypothetical chart based on the inventor's approximation of anappropriate correlation.

A correlation chart for an actual installation can be made in thefollowing way. First, after selecting the desired operating temperature,one finds the motive pump speed that corresponds to that temperature.This becomes the target pump speed. When the reactor temperature risesabove the desired temperature, the PLC must be programmed to decreasethe speed of the motive pump and vice versa. Generally, the maximumacceptable operating temperature will correlate to the lowest acceptablepump speed. Though the exact pump speeds associated with temperatureshigher or lower than the desired temperature are somewhat subjective, itis preferable to have a series of possible pump speeds that correspondwith the range of possible operating temperatures rather than having thepump run at its highest or lowest speeds in response to a variation fromthe desired temperature. This is preferable because it is easier on thesystem and more energy efficient.

FIG. 5 shows, a foam transfer pipe 67 employed in a treatment reactor.In the embodiment shown in FIG. 5, the top opening 69 includes a foamcollector 71 which opens to a layer of foam floating on top of asolution in the treatment reactor. The top of the foam 77 and the top ofthe solution 79 are shown in a general, approximate fashion in FIG. 5.Foam is shown entering the top opening 69 through the foam collector 71.As the foam collects in the foam collector 71 and in the foam transferpipe 67, a foam head pressure builds and forces the foam through thefoam transfer pipe 67. This foam head pressure also exerts a force thatmay rupture foam bubbles. Foam head pressure may be measured in feet.The amount of foam head pressure exerted on the foam depends on thedensity of the foam itself. Generally, the layer of foam is generated bytreatment of the solution or other reaction processes that take place inthe reactor. Continuous foam production by a reaction process in atreatment reactor can produce the static energy sufficient to move thefoam through the upper portion of the foam transfer pipe 69.

The foam collector 71 may be any shape, though a conical shape ispreferred. The top of a conical foam collector 71 is referred to as aweir. A longer weir length or diameter will increase the ability of thefoam collector to collect foam and direct it into the foam transferpipe. As a result, the foam collector 71 may be a shorter distance abovethe top of the solution, or the foam layer may be smaller, while stillcreating sufficient static foam head.

The foam transfer pipe 67 may be circular, square or another shape. InFIG. 5, the foam transfer pipe 67 is circular and has a first diameter,and the bottom opening 73 has a second diameter. The foam transfer pipe67 tapers so that the second diameter is smaller than the firstdiameter.

The foam transfer pipe 67 shown in FIG. 5 includes a static mixer 75. Astatic mixer can impart dynamic mixing action to a fluid as that fluidpasses by. The static mixer shown in FIG. 5 is expected to cause thefoam to mix in a swirling motion. The static mixer may be immobile,thereby reducing or eliminating the moving parts and energy requirementsand costs for controlling foam. A preferred form of static mixercomprises at least one helix-shaped protrusion (as shown in FIG. 5). Thestatic mixer shown in FIG. 5 comprises a helix-shaped flat protrusionrunning along the interior surface of a foam transfer pipe, although thestatic mixer need not be attached to the foam transfer pipe. That is,the static mixer may be free-standing inside the foam transfer pipe. Asa foam or other fluid passes by this helix-shaped protrusion, the foamor fluid may swirl or shear, which may thereby rupture bubbles thatcomprise foam. Other types of static mixers include off-hatching orspirals of any configuration running through the pipe. In a broad sense,a static mixer may be any impediment to fluid flow in the foam transferpipe that creates a sufficient dynamic mixing action to rupture foambubbles.

The foam transfer pipe 67 is fluidly connected to an outer nozzle 81 ofa jet aeration device of the type shown in FIGS. 1 and 3. However, thisparticular outer nozzle is dedicated to the foam transfer pipe and isnot connected to an air header. A suction, vacuum or draw is created inthe interior of the dedicated outer nozzle 81 by the liquid flow passingthrough the inner nozzle 7. (A higher liquid flow generates highersuction or draw.) The liquid flow through the jet aeration devicescreates a venturi action that pulls foam through the bottom opening 73into the dedicated outer nozzle 81. The foam head pressure enhances andworks additively with the venturi action and allows the venturi to movea higher volume of foam. Thus, the foam transfer pipe uses these twoforces to move foam. Through the dedicated outer nozzle 81, the foam(which is no longer entirely foam but now at least in part a liquid) isinjected back into the solution to be treated. Although it is believedthat most or all foam bubbles will be ruptured after passing through thestatic mixer, any remaining bubbles will likely be ruptured by theadditional shear generated in the jet aeration system. Furthermore, therecirculation of foam back into the reactor can allow materials to bebroken down into even simpler compounds which may eventually loose theirability to generate new foam bubbles.

Just as the liquid velocity passing through the inner nozzle 7 of thejet aeration system can control temperature by controlling theexothermic reaction rate, so can the liquid velocity control the foamlevel. By reducing the liquid velocity rate, less foam will begenerated.

The amount of pressure required to rupture or collapse a foam bubble isdefined by the surface tension of the bubble. Foam head pressure may besufficient to rupture the foam bubble in some cases and return the foamto liquid form. In such cases, a static mixer may not be required. Inother cases, it will be necessary to supply a dynamic force in additionto static head pressure to rupture foam bubbles. It is theorized that astatic mixer 75 can supply this dynamic rupturing force by creating apressure drop on the foam and by creating a dynamic movement on thefoam, thereby adding shear force to the already existing head pressureforces. The static mixer can also assure that the foam will not vortexwhile passing through the foam transfer pipe 69.

The foam transfer pipe 69 may be adjustable in height so that itsoperation is not limited to a particular level of solution or foam. Oneway to make the foam transfer pipe 69 adjustable is by making it atelescoping pipe.

The foam control method and apparatus described herein are not limitedto use with jet aeration systems. They can be used with other venturisystems or other systems that generate a vacuum, suction or draw.However, jet aeration systems, are preferred because of the relativelyhigh level of draw they can generate. Furthermore, the foam controlmethod and apparatus are not limited to autothermal thermophilic aerobicdigestion of biosolids; they may be used in connection with mesophilicaerobic digestion of biosolids; anaerobic mesophilic or thermophilicdigestion of biosolids; mesophilic or thermophilic biological treatmentof soluble organic compounds, treated by aerobic or anaerobictechnology; treatment processes in the chemical, petrochemical orpharmaceutical industries; and any other process that requires mixingand generates foam.

What is claimed is:
 1. A process for the aerobic treatment in abiosolids treatment reactor of a biosolids solution comprising theproducts of waste water treatment and thermophilic bacteria capable ofdigesting mesophilic bacteria, said process comprising: (a) mixing aportion of the biosolids solution with an oxygen-containing gas streamusing a jet aeration device; (b) monitoring at least one physicalproperty indicative of oxygen demand of the biosolids solution; (c)adjusting the mixing of biosolids solution with the oxygen-containinggas stream by the jet-aeration device such that sufficient oxygen issupplied to satisfy oxygen demand, wherein the monitoring step comprisesmonitoring the temperature and the oxygen/reduction potential of thebiosolids solution.
 2. An apparatus for aerobic treatment of waste watertreatment biosolids comprising: a reactor having an inlet for theintroduction of biosolids; a jet aeration device within the reactor;means for automatically sensing and controlling the temperature of thesolution within the reactor; and means for automatically sensing andcontrolling the oxygen/reduction potential of the solution within thereactor, both of said means being operatively attached to jet aerationdevice so that based on the temperature and oxygen/reduction potentialof the biosolids solution, the means will adjust the oxygen supply tothe reactor.
 3. A process for the aerobic treatment in a biosolidstreatment reactor of a biosolids solution comprising the products ofwaste water treatment and thermophilic bacteria capable of digestingmesophilic bacteria, said process comprising: (a) mixing a portion ofthe biosolids solution with an oxygen-containing gas stream using anaeration device; (b) monitoring the temperature and the oxygen/reductionpotential of the biosolids solution; (c) adjusting the mixing ofbiosolids solution with the oxygen-containing gas stream by the aerationdevice such that sufficient oxygen is supplied to satisfy oxygen demand.4. An apparatus for aerobic treatment of waste water treatment biosolidscomprising: a reactor having an inlet for the introduction of biosolids;an aeration device within the reactor; means for automatically sensingand controlling the temperature of the solution within the reactor; andmeans for automatically sensing and controlling the oxygen/reductionpotential of the solution within the reactor, both of said means beingoperatively attached to the aeration device so that based on thetemperature and oxygen/reduction potential of the biosolids solution,the means will adjust the oxygen supply to the reactor.
 5. A process forthe aerobic treatment in a biosolids treatment reactor of a biosolidssolution comprising the products of waste water treatment andthermophilic bacteria capable of digesting mesophilic bacteria, saidprocess comprising: (a) mixing a portion of the biosolids solution withan oxygen-containing gas stream using an aeration device; (b) monitoringthe temperature and the oxygen/reduction potential of the biosolidssolution; (c) when the oxygen/reduction potential of the biosolidssolution is below a predetermined level, reducing the airflow throughthe aeration device into the biosolids solution; (d) when thetemperature of the biosolids solution is below a predetermined level,increasing the amount of shear generated through the aeration device. 6.An apparatus for aerobic treatment of waste water treatment biosolidscomprising: a reactor having an inlet for the introduction of biosolids;an aeration device within the reactor; a pump operatively connected tothe aeration device to supply oxygen-containing gas to the reactorthrough the aeration device; a temperature sensor within the reactor; anoxidation/reduction potential sensor within the reactor; a pumpcontroller that is connected to, the temperature sensor and theoxygen/reduction potential sensor, the pump controller being operativelyattached to the aeration device so that based on the temperature andoxygen/reduction potential of the biosolids solution, the pumpcontroller will adjust the supply of oxygen-containing gas to thereactor.